Recently, rare earth-doped silica or silica-based materials have most widely been used to manufacture waveguide amplifiers. However, because a light source directly pumps the rare earth atoms, and because rare earth atoms have narrow absorption bands and small absorption cross sections of the order of 4×10−21cm2, in order to pump such a waveguide amplifier, an end fire technique must be used for coupling light into the waveguide through an optical fiber using a high-priced laser as the light source.
In case of doping silicon nanoclusters with rare earth atoms in such a waveguide, an electron-hole combination formed in the nanocluster can give rise to excitation of the rare earth atoms. The effective excitation cross section is approximately 1×10−15cm2. Considering that the concentration of the silicon nanocluster is generally 1×1018 cm3, the above value corresponds to an absorption depth of 10 μm or less. In this regard, if a waveguide amplifier made of the aforementioned two materials is subjected to a top pumping process, in which a light source is positioned above the waveguide, although no such attempts have been made, it may have an enhanced efficiency. In particular, because there are no particular limitations to a light source so long as the light source can generate carriers in the silicon nanocluster, a low-priced wide band light source such as LED can be used instead of a high-priced laser. However, contrary to the end fire technique for coupling almost all light from a light source into a waveguide through an optical fiber, the top pumping process, in which a light source is positioned above a waveguide, cannot allow all light from the light source to enter the waveguide. As a result, an actual pump power is substantially decreased relative to that from the light source, thereby lowering the excitation efficiency.